A scanning electron microscope (hereinafter, referred to as SEM) which is a representative charged particle beam apparatus is used for observation of defects or faults in a semiconductor pattern. In recent years, with miniaturization of semiconductor processes, in the SEM, observation at higher magnification is required. The SEM emits an electron beam (primary electron beam) onto the surface of a sample to be observed while scanning in a two-dimensional shape, and sets the intensity of a secondary electron and a reflection electron (hereinafter, referred to as secondary particle) secondarily generated from the sample as a luminance modulation input of an image scanned and generated in synchronization with scanning of the primary electron beam, thereby obtaining an image (SEM image) to be observed of the sample surface. At this time, a sample surface to be observed is selected by moving a stage with a sample mounted on the stage provided in the SEM. Usually, an operator operates operation input means, such as a trackball, while viewing the SEM image, and moves the stage until an intended sample surface portion is obtained as a SEM image. In this case, in order for the operator to easily recognize a current observation position, it is preferable to move the stage at constant speed such that the SEM image moves at constant speed. When observing a sample, such as a semiconductor wafer, in which the same fine patterns (cells) are continuous, a method which measures the number of cells (cell count) passing across a SEM screen using visual observation or image processing, and determines whether or not an intended portion has been reached is used. In this case, it is necessary to move the stage at constant speed such that the SEM image is moved at constant speed without oversight of cells. As described above, not only positioning with a high degree of accuracy is required so as to observe at high magnification, but speed stability is also required.
In a conventional stage mechanism, a positioning apparatus has been widely used, the positioning apparatus in which the ball screws are rotated by a stepping motor to move a stage through the nuts attached to the ball screws. However, in this mechanism, errors in the engagement between the ball screw and the nut, elastic deformation of the ball screw, errors in rotation of the stepping motor, etc. cause occurrence of so-called backlash immediately after the time when the movement direction of the stage is reversed, the stage is not moved in response to a command for movement of the stage, or the movement is smaller than a value by the command even if the stage is moved. As a result, the positioning with a high degree of accuracy cannot be obtained, thereby leading to one of causes which hinders the operability of the stage.
In order to solve such errors in the positioning, there has been disclosed a method in Japanese Patent Application Laid-Open Publication No. 2004-288918 (Patent Document 1) in which an ultrasonic motor composed of a piezoelectric element (electrostrictive element) is attached directly to a lower surface of the movable table to directly drive the stage, instead of a conventional stepping motor. Patent Document 1 discloses a drive system in which a piezoelectric element is pressed against a drive transmission surface provided to a lower surface of the stage, and accordingly, the drive transmission surface is linearly moved in the direction of movement of the stage with use of the shearing deformation of the piezoelectric element when a power voltage is applied, thereby driving the stage.